1. Field of the Invention
The present invention relates to frequency converters and radio communications systems employing the frequency converter and, more particularly, to a frequency converter and a radio communication, in which a received radio frequency signal is converted into an intermediate frequency signal and an intermediate frequency signal to be transmitted is converted into a radio frequency signal.
2. Description of the Related Art
There is a growing demand for a high-speed network as data communications rapidly gain widespread use. High-speed communications provided by wired networks are still very expensive for individual users, and local radio communications networks providing a low-cost service are now actively studied and developed. Such a local radio network uses radio frequency bands, such as quasi millimeter waves (20 GH–30 GHz) or millimeterwaves (30 GH–300 GHz), capable giving a high antenna gain with a small antenna. In the local radio network, a hub station, for example, installed in a telephone exchange station, provides high-speed two-way data communications service or local TV phone service to a plurality of (user) subscriber stations within a predetermined coverage area.
When radio communications are performed using radio frequency signals in the quasi millimeter band or the millimeter band, an intermediate frequency signal of several tens to several hundreds of megahertz, rather than radio frequency signals, is subjected to a receiving process including an isolation decoding step, a transmitting process including coding and synthesis steps, and an amplification of signals. The cost of a circuit arrangement required for the receiving and transmitting processes and the signal amplification is thus reduced. FIG. 16 shows a frequency converter, which converts a radio frequency signal into an intermediate frequency signal or an intermediate frequency signal into a radio frequency signal.
The frequency converter is installed in a subscriber station, for example, and receives radio waves, transmitted from a hub station, through its receiving antenna 1001. Out of the radio waves received by the receiving antenna 1001, radio frequency RF(RX) signals of interest for reception in a plurality of frequency channels in a range of 22.6 GHz–23.0 GHz are extracted by a bandpass filter 1002.
The radio frequency RF(RX) signal extracted through the bandpass filter 1002 is amplified to an appropriate level by a low noise amplifier 1003, and is then mixed with a TX/RX local oscillation frequency signal LO1, for example, a 21 GHz signal, by a mixer 1004.
The local oscillation frequency signal LO1 is generated by a phase-locked oscillator 1100.
The phase-locked oscillator 1100 includes a phase-locked loop including a counter circuit 1102, a frequency comparator 1103, a loop filter 1105, and a voltage-controlled oscillator 1106, and frequency multipliers 1107 and 1109.
In the phase-locked oscillator 1100, a signal output by the voltage-controlled oscillator 1106 is frequency-divided, for example, by 175, by the counter circuit 1102. The frequency comparator 1103 compares a signal output by the counter circuit 1102 to a reference signal, for example, a 10 MHz reference signal supplied by a reference oscillator 1204 employing a highly stable crystal oscillator. A voltage, corresponding to the difference between the two signals, is then amplified by the loop filter 1105 in appropriate frequency characteristics. The voltage output from the loop filter 1105 is fed back to a control input of the voltage-controlled oscillator 1106.
A 1.75 GHz signal output by the voltage-controlled oscillator 1106 is frequency-multiplied by four times by the frequency multiplier 1107, becoming a 7.0 GHz signal. The remaining signals contained in the output of the frequency multiplier 1107 are filtered out by a bandpass filter 1108.
The signal output by the bandpass filter 1108 is further frequency-multiplied by three times by the frequency multiplier 1109, becoming a 21.0 GHz signal. The remaining signals contained in the output of the frequency multiplier 1109 are filtered out by a bandpass filter 1110.
In this way, the phase-locked oscillator 1100 results in the signal having the local-oscillation frequency LO1 (21 GHz) at the same frequency accuracy level as that provided by the highly stable reference oscillator 1204.
The local-oscillation frequency signal LO1 is output by the bandpass filter 1110, and is amplified by an amplifier 1112, and is then received by the RX mixer 1004.
An output of the mixer 1004 contains the frequency components of the sum of, and the difference between, the radio frequency RF(RX) signal and the local-oscillation frequency LO1 signal. The difference between the two signals, i.e., a signal in an intermediate frequency band IF1(RX) of 1.6 GHz–2.0 GHz, is extracted by the bandpass filter 1005, is amplified by an amplifier 1006, and is fed to an RX mixer 1007.
The RX mixer 1007 mixes the signal in the intermediate frequency band IF1(RX) and a local-oscillation frequency LO2 signal, for example, a 1.1 GHz signal supplied by a phase-locked oscillator 1200.
The local-oscillation frequency LO2 signal is generated by the phase-locked oscillator 1200.
The phase-locked oscillator 1200 includes a phase-locked loop including a counter circuit 1202, a frequency comparator 1203, a loop filter 1205, and a voltage-controlled oscillator 1206, and the high-accuracy reference oscillator 1204 employing a crystal oscillator.
In the phase-locked oscillator 1200, a signal output by the voltage-controlled oscillator 1206 is frequency divided, for example, by 110, by the counter circuit 1102. The frequency comparator 1203 compares a signal output by the counter circuit 1202 to a reference signal, for example, a 10 MHz reference signal supplied by the reference oscillator 1204. A voltage, corresponding to the difference between the two signals, is then amplified by the loop filter 1205 in appropriate frequency characteristics. The voltage output from the loop filter 1205 is fed back to a control input of the voltage-controlled oscillator 1206.
In this way, the phase-locked oscillator 1200 results in the signal having the local-oscillation frequency LO2 (1100 MHz) at the same frequency accuracy level as that provided by the highly stable reference oscillator 1204.
The output of the RX mixer 1007 contains frequency components of the sum of, and the difference between, the signal in the intermediate frequency band IF1(RX) and the local-oscillation frequency LO2 signal. The difference between the two signals, i.e., a signal in an intermediate frequency band IF2(RX) of 500 MHz–900 MHz, is extracted through the bandpass filter 1008.
The signal in the intermediate frequency band IF2(RX), picked up by the bandpass filter 1008, is amplified by an amplifier 1009, is fed to a diplexer 1010, and is then fed to a demodulator (not shown) via an IF cable.
The signal in the radio frequency band RF(RX) thus received is converted into a signal in an appropriate intermediate frequency band IF2(RX).
The signal in the intermediate frequency band IF2(TX), for example, 10 MHz–60 MHz, supplied by a modulator (not shown), is received from the diplexer 1010 via the IF cable, is amplified by an amplifier 1012, and fed to a TX mixer 1013. The RX intermediate frequency IF2(RX) and the TX intermediate frequency IF2(TX) are assigned in separate frequency ranges.
The TX mixer 1013 mixes the signal in the intermediate frequency band IF2(TX) with the signal having the local-oscillation frequency LO2 output by the phase-locked oscillator 1200.
The output of the TX mixer 1013 contains the signals of the sum of, and the difference between, the signal in the intermediate frequency band IF2(TX) and the local-oscillation frequency LO2 signal. The signal of the sum of the two signals, i.e., a signal in an intermediate frequency band IF1(TX) of 1.11 GHz–1.16 GHz, is extracted by a bandpass filter 1014.
The signal in the intermediate frequency band IF1(TX), extracted by the bandpass filter 1014, is amplified by an amplifier 1015, and is then fed to a TX mixer 1016.
The TX mixer 1016 mixes the signal in the intermediate frequency band IF1(TX) with the signal in the local-oscillation frequency LO1 output by the phase-locked oscillator 1100.
The output of the TX mixer 1016 includes the signal components of the sum of, and the difference between, the signal in the intermediate frequency band IF1(TX) and the signal having the local-oscillation frequency LO1. The signal of the sum of the two signals, i.e., a signal in a radio frequency band RF(TX) of 22.11 GHz–22.16 GHz, is extracted by a bandpass filter 1017.
The signal in the radio frequency band RF(TX), extracted by the bandpass filter 1017, is amplified to an appropriate level, and is fed to a transmitting antenna 1019. The corresponding radio wave is then transmitted through the transmitting antenna 1019 to the hub station.
In this way, a signal in the appropriate intermediate frequency band IF2(TX) is frequency converted into a signal in the radio frequency band RF(RX) and is then transmitted.
The conventional frequency converter employs the phase-locked oscillators 1100 and 1200 to respectively generate a plurality of signals in the local-oscillation frequencies LO1 and LO2, shared by the transmitter part and the receiver part.
The phase-locked oscillators having a crystal oscillator working in the quasi millimeter band or the millimeter band are generally costly. They are complex and bulky, requiring a substantial maintenance cost, and consume much power, requiring a substantial operating cost. As a result, if the frequency converter is built in a subscriber station in a local radio network (a radio communications system), the cost of each individual subscriber's subscriber station increases much.
When the local-oscillation frequency is generated using the phase-locked oscillator, the frequency accuracy inevitably degrades in proportion to the ratio N of the frequency of the output signal to the frequency of the reference signal in the crystal oscillator, and a phase noise inevitably increases in proportion to the square of N. As the local-oscillation frequency becomes higher, the attained accuracy level of the output frequency and the level of the phase noise are limited more.
For example, when the frequency accuracy of the 10 MHz reference oscillator is +/−10 Hz (1E-6) and the phase noise is −120 dBc/Hz at an offset of 1 kHz, N=21 GHz/10 MHz=2100 at a frequency of 21 GHz. The attained frequency accuracy level is +/−21 kHz, and the phase noise level is −54 dBc/Hz at an offset of 1 kHz because 20 log 2100=66 dB.
If the local-oscillation frequency becomes higher, a frequency multiplier may be required. The frequency multiplier can work as a source of unwanted radiated signals, degrading spurious characteristics of the frequency converter.
These degraded characteristics lead to a drop in the utilization of frequencies when communications are performed using frequency division multiplexing.